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P P h o
s B e s t
Man a g e m e n
t P r a c
t i c e
s
ProtectingWater Quality
veloped by
uthern Extension
search Activity-
ormation Exchange
oup-17
p://sera17.ext.vt.edu/
thor’s emaile.hawkins@va.
da.gov
ting and Design:
rbes Walker
anda Russell
ry Dagnan
ne Dalton
iversity of
nnessee Extension
Definition: A sediment retention
and nutrient removal
treatment system that
uses natural chemical,
physical and biological
processes involving wet-
land vegetation, soils and
their associated microbi-
al populations to improve
water quality.
Purpose:Excessive phosphorus
is contributed by waste-
water effluent, urban
runoff and agricultural
point and nonpoint sourc-
es. Phosphorus is most
often the limiting nutri-ent causing eutrophica-
tion of surface waters.
Treatment wetlands are
an ecologically engi-
neered system that can
be used in a full-scale
waste treatment system
to remove phosphorus.
Phosphorus retention in
wetland systems occurs
via sorption, precipita-tion and incorporation,
balanced by immobiliza-
tion and, most impor-
tantly, sedimentation.
Research has shown that
treatment wetlands can
be more affordable, re-
liable and practical to
build and operate, as well
as more sustainable, than
conventional treatment
technology.
Treatment wetlands
are designed to use wa-
ter quality improvement
processes occurring in
natural wetlands, includ-
ing high primary produc-
tivity, low-flow conditions
and oxygen transport
to anaerobic sediments.
Treatment wetlands
also help to minimize
odor problems, reduce
labor costs associated
with hauling and apply-
ing effluent and provide
aesthetic and wildlife
benefits.
How Does This Practice Work?
There are two types
of constructed treatment
wetlands: surface flow
systems where wastewa-
ter flows as sheet flow
above the ground surface
and subsurface flow sys-
tems in which water flowshorizontally and verti-
cally through porous me-
dia. Which system is used
and the design of each is
based on influent waste-
water characteristics,
pollutant removal goals,
wetland sizing equations
and methods, amount
and timing of wastewa-
ter flows, land area, cost
and availability of plants,
media cost and perme-
ability, suitable soils,
discharge standards and
regulatory requirements.
Depending on the
source of the wastewater
(nonpoint agricultural
runoff, confined animal
wastewater, etc.), pre-
treatment is usually nec-
essary due to the high
levels of organic carbon,
nitrogen and solids in the
wastewater. Otherwise,
the wetland system could
be overloaded with oxy-
gen-demanding pollut-ants and solids that would
cause the wetland plants
to die. Pre-treatment
facilities can include la-
goons, storage ponds or
solids separators.
The key to designing
a wetland system to the
right size is to know the
constituency of the pol-
lutants, the load concen-tration, the desired efflu-
ent concentration and the
sources and the amounts
of water being added to
the wastes, yielding the
total wastewater volume
in the wetland system.
For example, the major
water sources for an ag-
Constructed Treatment
Wetlands Author: Julie Hawkins, USDA-NRCS, Richmond, Virginia
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ricultural system are usually
flushwater to remove manure
from buildings, water for clean-
ing milking and milk-process-
ing facilities, rainfall runoff
from roofs and open lots and
direct rainfall on pretreatment
facilities and the wetland.
The fate of treated watermust be determined when the
wetland is designed. Various
options include using the water
to irrigate crop areas, for recy-
cling as flushwater, discharg-
ing to a surface water or cre-
ating additional wetland and
aquatic habitat on the farm.
Where This Practice Appliesand Its Limitations:
Treatment wetlands have
been used as a water quality
improvement technique in di-
verse climate regimes and with
varying types of wastewater.
Consideration of all effluent
constituents and loading rates
is very important. High biologi-
cal oxygen demand (BOD), a
measure of organic compound
load, above the accepted
range of 112 kg/ha/year willlikely produce microbiologi-
cal processes that are entirely
anaerobic and may lead to the
collapse of the entire wetland
system. High ammonia can
lead to ammonia toxicity, and
hence kill plants in the wet-
land system. Plants have vari-
ous ranges of ammonia levels
they can tolerate; these shouldbe considered during design.
Low pollutant loading rates
for any wastewater constitu-
ent may not show any removal,
as wetlands systems contain
background levels of nutrients,
carbon and metals.
Over the long term, phos-
phorus storage in wetland eco-
systems is ultimately limited
by sedimentation. In surface
flow wetlands, the amount of
phosphorus sorption sites is
based on the amount of cal-
cium (Ca), iron (Fe) and/or
aluminum (Al) in the soil
substrate. These exchange
sites may become saturated.
Additions of Al and Fe may be
feasible, but would need to be
added upstream from the wet-
land, as chemical contacting
is not efficient. On the otherhand, subsurface flow wetlands
can use media high in Al and/
or Fe or Ca to provide a high
amount of sorption sites.
The land area required to
meet adequate detention times
and treatment goals may be
limiting and unavailable. In
general, these systems should
be engineered and constructedin uplands, outside U.S. waters,
unless the source water can be
used to restore a degraded or
former wetland. Federal, state,
tribal and/or local regulations
may be applicable.
A wetland treatment tech-
nology may not be the most
cost-effective, environmentally
sensitive or technically reliable
process for a given wastewater
or project location. It is impor-
tant to consider and compare
all forms of treatment for the
wastewater situation.
Effectiveness:When included as a compo-
nent of a farm-wide waste-man-
agement plan, treatment wet-
lands are effective. However,
they may prove ineffective
without pretreatment of thewastewater containing high
pollutant loads. The feasibil-
ity of treatment wetlands var-
ies with waste characteristics,
hydraulic retention time and
climate. Extremes in meteo-
rological conditions should be
considered. Treatment func-
tion still occurs at lower tem-
peratures, although at reduced
rates, and may affect the de-tention time. High-precipita-
tion events or snow melts may
cause unplanned temporary
exceeding of water storage and
treatment capacity and should
be considered and planned for
during design.
Average reduction in
phosphorus from wetland sys-
tems at dairy, cattle, swine,
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poultry and aquaculture sites
was determined to be 42 per-
cent (range of 20-90 percent).
However, increasing pretreat-
ment or wetland area would in-
crease this percentage to equal
to or above that of conventional
phosphorus treatment (57 per-
cent). In comparison to conven-tional spray fields, treatment
wetlands are estimated to
remove three to seven times
more phosphorus, depending
on hydraulic residence time
and vegetation planted.
This is a fairly new tech-
nology and is continuously de-
veloping. There are still gaps
in the understanding of these
systems that restrict our ability
to achieve predictable and sus-
tained levels of water quality
improvement. Specifically, sol-
uble phosphorus may increase
as water moves through the
wetland system, particularly as
vegetation starts to mature. If
reducing soluble phosphorus is
the objective of a specific proj-
ect, consider reviewing other
best management practices as
a better solution to removingsoluble phosphorus than con-
structed treatment wetlands.
Cost of Establishing andPutting the Practice in Place:
A variety of factors includ-
ing detention time, treatment
goals, depth of media, type of
pretreatment, number of wet-
land cells, source and availabil-
ity of gravel media and terrain(relatively flat topography rec-
ommended) will affect the cost
of wastewater treatment wet-
lands. The capital costs of con-
structed treatment wetlands
can be separated into land,
excavation, liners, gravel (sub-
surface flow systems), plants,
distribution and control struc-
tures and fencing. The single
most important factor affectingthe capital cost is the cost of
gravel. Second is the cost of the
liner material.
The cost of a typical 50,000
ft2 (4650 m2) subsurface con-
structed wetland is $122,000.
In general, the gravel was
43 percent of the total cost.
Gravel, liner ($32,000 −
$100,000 per ha) and excava-
tion followed at 16, 11 and 10
percent, respectively. Plumbingand control structures and
other minor items (e.g., fenc-
ing) made up the remaining
percentages at 6, 6 and 8 per-
cents, respectively. The costs
are relative using a 2-foot (0.6
m) depth and typical unit pric-
es that can be found in many
places in the U.S.
In general, the median
cost of surface and subsurfaceflow wetlands is $20,000 per
acre ($47,000 per hectare) and
$145,000 per acre ($378,000
per hectare), respectively.
When compared to convention-
al treatment methods, treat-
ment wetlands systems prove
to be up to 30 percent cheaper
when considering lifespan and
replacement values.
To reduce earthwork costs,the designer can place smaller
berms between adjacent cells
and lower length-to-width
ratios. When possible, the de-
signer should use gravity flow,
minimize pipeline distance
and size, use open vegetated
conveyance channels and radi-
ating cells to minimize the cost
and complexity of the inlet dis-
tribution system.
In general, constructedtreatment wetlands are a sim-
pler alternative and require
more land and less energy to
install and operate. As a return
to users, lower utilities to oper-
ate these systems yield lower
total costs. Although land
requirements for constructed
treatment wetlands are often
more than conventional sys-
tems, the difference in thecapital costs will be repaid by
lower operation and mainte-
nance costs in a relatively short
period of time.
Operation and Maintenance:Operational costs can be
divided into testing, water
level adjustment, weed control,
flow distribution and level
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adjustment sumps. Operation
and maintenance costs range
from $400 per year for surface
flow systems to $1000 to $2000
per year for subsurface flow
systems.
A long-term operations,
maintenance, monitoring and
funding plan that identifiesthe individuals responsible for
maintenance and monitoring of
the project, their responsibili-
ties and funding mechanisms
must be created.
Constructed treatment
wetlands are operated and
maintained by controlling
the water’s quality, quantity,
depth and flow rate. Regular
inspections of the constructed
treatment wetland should be
made that are case-specific
and dependent on maintenance
activities. These include check-
ing weir settings and the inlet
and outlet structures, cleaning
off surfaces where solids and
floatable substances have accu-
mulated to the extent that they
may block flows, removing nui-
sance species, maintaining the
appearance and general statusof the vegetation and remov-
ing sediment accumulations in
forebays. Besides structures,
periodic monitoring of param-
eters such as BOD5, TSS, nitro-
gen and phosphorus will guide
the wetland operation.
The major concern in op-
erating and maintaining con-
structed treatment wetlands
is the clogging of the gravelmedium. Limited data yield a
range of 33 to 150 years, de-
pending on the accumulation
rate of senesced plant material
and sediment. A conservatively
estimated minimum of 33
years will aid in the planning
process for replacing such
items as gravel.
This project was funded in part under an agreement with the USDA-NRCS.
Adaptive management and
manipulation are key engineer-
ing factors that must be incor-
porated into design and opera-
tion of treatment wetland sys-
tems. It is necessary to achieve
and exceed performance objec-
tives within a minimal space,
with minimal effort and exter-nal resources and with maxi-
mum reliability.
References: Campbell, Craig S. and
Michael H. Ogden. 1999.
Constructed Wetlands in the
Sustainable Landscape. John
Wiley & Sons, Inc. New York,
NY, 270 p.
CH2MHill. 2001. Constructed
Wetlands and Wastewater
Management for Confined
Animal Feeding Operations.
CH2MHill, Gainesville, FL,
23 p.
Clarke, Ernest and Andrew H.
Baldwin. 2002. Responses of
wetland plants to ammonia
and water level. Ecological
Engineering 18:257-264.
Cronk, Julie K. 1996.
Constructed wetlands to treat
wastewater from dairy and
swine operations: a review.
Agriculture, Ecosystems and
Environment 58: 97-114.
DeBusk, Thomas A., James
E. Peterson and K. Ramesh
Reddy. 1995. Use of aquatic
and terrestrial plants for removing phosphorus from
dairy wastewaters. Ecological
Engineering 5: 371-390.
Kadlec, R.H. and R.L. Knight.
1996. Treatment Wetlands.
Boca Raton, FL, CRC Press/
Lewis Publishers.
U.S. Environmental Protection
Agency. 2000. Guiding
Principles for Constructed
Treatment Wetlands: Providing
for Water Quality and Wildlife
Habitat . Washington, DC,
EPA 843-B-00-003, 41 p.
U.S. Department ofAgriculture, Natural
Resources Conservation
Service. 2002. Chapter
3: Constructed Wetlands.
National Engineering
Handbook, Environmental
Engineering. Washington,
DC, 74p.
Werker, A.G., J.M. Dougherty,
J.L. McHenry and W.A.
Van Loon. 2002. Treatment
variability for wetland waste-
water treatment design in
cold climates. Ecological
Engineering 19:1-11.
White, Jay S., Suzanne E.
Bayley and P. Jeff Curtis.
2000. Sediment storage of phos
phorus in a northern prairie
wetland receiving municipal
and agro-industrial wastewa-ter . Ecological Engineering
14:127-138.
For Further Information: Contact your local conservation
district, USDA-NRCS or
Cooperative Extension Service
office.